The present invention relates to a method of forming passivation films which serve to protect circuits and/or circuit elements on semiconductors and IC (integrated circuit) chip surfaces. In particular, the present invention relates to a method of forming films that feature extremely high levels of reliability and production rate and which are suitable for use as passivation films on high-integration chips incorporated in LSIs (large-scale integrated circuits), VLSIs (very large-scale integrated circuits) and microcomputers.
In the fabrication of semiconductor devices, in particular resin-molded semiconductor devices, inorganic passivating films are deposited to protect circuits on semiconductor chip surfaces from handling and contamination during processing. Such passivating films are typically made of phosphosilicate glasses (PSG) or silicon nitride (Si.sub.3 N.sub.4) in single layers or in two stacked layers of PSG and Si.sub.3 N.sub.4.
In conventional IC fabrication, a prepared wafer provided with the necessary circuit or circuit elements on the surface is subsequently protected by a thin passivating film which is formed of PSG or Si.sub.3 N.sub.4 or a laminate thereof by sputtering or other suitable film-deposition techniques. In these cases, the bonding pads which should not be covered would also be covered with the passivating film. Therefore, in order to open windows in the areas corresponding to the bonding pads, a photoresist is applied to the same wafer, which, after pre-baking, is positioned on an aligner and the areas above the windows are exposed to ultraviolet radiation (if the resist is positive-working) to make them more soluble than unexposed areas. Thereafter, the resist on the windows is solubilized and stripped by an etching solution and the thin PSG or Si.sub.3 N.sub.4 film is removed by dry plasma etching. In addition, the resist remaining in the areas other than those on the windows is solubilized and stripped by a rinse solution, forming the desired inorganic passivation film, such as PSG and/or Si.sub.3 N.sub.4.
But, the integrity of the so formed passivation film is not complete in the strict sense of the term because upon closer examination it has pinholes and/or microcracks at stepped portions. In order to increase its reliability, the passivation film is usually covered with an organic thin film. However, if this is done by spin coating as a step in wafer preparation, windows must also be opened in the areas corresponding to bonding pads and procedures which are essentially the same as those employed after the formation of the inorganic passivating film must be repeated: application of a polyimide (PI), silicone or epoxy varnish by spin coating; baking; application of a photoresist (by spin coating) and pre-baking; setting on an aligner for aligning purposes; and exposure to ultraviolet radiation (in the areas on the windows such as for bonding pads if the photoresist is positive-working) so as to make the selected areas more soluble than the other areas. Subsequently, the unwanted areas of the photoresist are solubilized and stripped by an etching solution. The etchant also serves to open windows in the overlying organic passivation film made of PI or silicone or epoxy varnish. The remaining resist film is solubilized and removed by a rinse solution, then the organic passivation film forming the desired overcoat is subjected to post-baking until it hardens completely. These procedures complete the formation of the overall passivating film.
Being formed by the method described above, the conventional passivating film has pinholes and microcracks at stepped portions. In addition, the thin inorganic passivation film is so brittle that cracks will easily develop upon receiving mechanical shock during assembly of parts or mechanical or thermal stresses during packaging with molding resins. This has been one of the major causes of lowered reliability of the final product.
As already mentioned, the reliability of the thin inorganic passivation film can be increased by protecting it with an organic thin film which is made from PI, silicone, epoxy or silicone ladder polymer by spin coating, screen printing, potting or any other suitable techniques. However, these approaches still involve such complicated and numerous steps that not only huge capital investment is required but a considerable drop also occurs in the production rate. In addition, the chance of wafer or chip damage during materials handling or dirt or dust buildup or subsequent fouling